Catalytic Hydrogenation of 5,6‐Dihydro‐4 H ‐1,2‐oxazines Bearing a Functionalized Methylene Group at C‐3

The catalytic hydrogenation of readily available methyl 2‐(5,6‐dihydro‐4 H ‐1,2‐oxazin‐3‐yl)acetates 6 has been studied. Dihydrooxazines 6 without an alkoxy substituent at C‐6 under mild hydrogenation conditions in methanol produce a dynamic mixture of enamines 7 and tetrahydro‐2‐furanamines 7′ (α + β). These products can be transformed into 1,4‐amino alcohols 8 under more robust hydrogenation conditions or into isomeric dihydrofurans 9 and 10 if the reduction is carried out in glacial acetic acid. Reduction of dihydrooxazines 6h , i , which possess an alkoxy substituent at C‐6, under similar conditions affords pyrrolidine derivatives 12 , 13 and 14 . A general mechanistic scheme for the hydrogenation reaction that involves an initial N–O bond cleavage has been suggested. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)